How advertising cookies let observers follow you across the web

A new paper from computer scientists at Princeton breaks down exactly how easy it is, even without the resources and access of the NSA. The researchers were able to reconstuct as much as 90% of a user’s web activity just from monitoring traffic to ad-trackers like Google’s DoubleClick. Crucially, the researchers didn’t need any special access to the ad data. They just sat back and watched public traffic across the network.